Electric vehicle transmissions having more than one gear have an advantage over one-gear transmissions in that they allow the electric machine to operate at an optimal number of revolution per minute, i.e. at the number of revolution per minute corresponding to the maximum efficiency of the machine, in a wider range of vehicle speed. Such an advantage is achieved both in the traction operating mode, in which the electric machine operates as a motor to generate the power required to drive the vehicle, and in the regeneration operating mode, in which the electric machine operates as a generator to convert the kinetic energy of the vehicle, upon deceleration, into electric energy.
A two-gear transmission for electric machines is known from EP-A-2 305 501. According to this known solution, the transmission comprises a primary shaft, a secondary shaft, a gear set for a first gear, or low gear, which includes a first driving gearwheel carried by the primary shaft and a first driven gearwheel carried by the secondary shaft and permanently meshing with the first driving gearwheel, and a gear set for a second gear, or high gear, which includes a second driving gearwheel carried by the primary shaft and a second driven gearwheel carried by the secondary shaft and permanently meshing with the second driving gearwheel.
The first driven gearwheel (or, alternatively, the first driving gearwheel) includes a hub rigidly connected for rotation with the respective shaft and a ring gear mounted around the hub so as to be able to rotate relative to the hub, an overrunning clutch being interposed between the ring gear and the hub to allow the transmission of the torque through the gear set of first gear only in the direction from the primary shaft to the secondary shaft. The transmission further includes a first coupling device arranged to connect the gearwheel to which the overrunning clutch is associated for rotation directly with the secondary shaft, so as to allow the transmission of the torque through the gear set of first gear also in the direction from the secondary shaft to the primary shaft, and a second coupling device arranged to connect the second driving gearwheel or the second driven gearwheel for rotation with the respective shaft.
The first coupling device includes a hub, which is rigidly connected for rotation with the secondary shaft, and a sliding sleeve, which is provided with first engagement teeth arranged to mesh with second engagement teeth of the gearwheel to which the overrunning clutch is associated. The sliding sleeve of the first coupling device is connected for rotation with the respective hub and is axially slidable relative to the hub between a disengagement position, in which the first and second engagement teeth are disengaged from each other, and an engagement position, in which the first and second engagement teeth mesh with each other, thereby connecting the ring gear of the gearwheel to which the overrunning clutch is associated for rotation with the secondary shaft.
In order to ensure correct operation of this known transmission, and more specifically of the overrunning clutch, it is necessary to properly set the plays inside the overrunning clutch and between the engagement teeth of the sliding sleeve of the first coupling device and of the ring gear of the gearwheel to which the overrunning clutch is associated, and more specifically the plays inside the overrunning clutch must be less than the plays between the aforesaid engagement teeth. It may therefore happen that if those plays are not ensured, due to manufacturing tolerances and/or deformations, the transmission will not operate correctly.